In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light but also electromagnetic radiation having a wavelength between 800 nanometres (nm) and 3000 nm.
Optical fibre based systems are the norm for high data-rate trunk and increasingly, local area networks (LAN) telecommunications. With 10 Gbit/s systems in the ascendant and 40 Gb/s on the near horizon, copper-based systems are not an option even for relatively short links; coaxial cable with losses around 3 dB/m at 30 GHz cannot compete with the fraction of a dB/km of optical fibre. There are also important issues of dispersion and cost.
High bit-rates and the need for wavelength finesse and stability preclude the use of direct laser modulation for such systems and external modulation of an extant laser beam is required. The present invention is concerned with a modulator for use in such a system. The concept of integrated optical (or ‘photonic’) circuits is not new but, until recently, commercial—and hence telecom systems—use was limited to relatively simple devices, primarily lithium-niobate modulators, which are available from several commercial sources.
The basic element of these modulator devices is the guided wave Mach-Zehnder interferometer. These devices can be regarded as a pair of parallel optical waveguides fed by a splitter and leading to a recombiner. The two parallel waveguides are formed of a material with electro-optic properties; that is a material whose refractive index can be varied in response to an electrical field (E-field) across the material. The speed of light in a material is inversely proportional to the refractive index, n, of the material through which the light is propagating.
Thus if the light passing through one of the parallel waveguides encounters a different refractive index, n, compared to light passing through the other, it is differentially delayed or shifted in phase. When the light from the parallel waveguides is suitably recombined the resultant coherent interference can be arranged to provide intensity modulation of the original light source.
Because the change in n is very rapid in a suitable electro-optic material, the modulator can be used to modulate at very high frequencies, up to beyond 100 GHz.
The free space optical wavelengths of interest, which are modulated using the present invention, are usually in the range 800 to 2000 nm.
Modulators based on Mach-Zehnder interferometers (MZI) have been developed in both the non-semiconducting ferro-electric materials such as lithium niobate and in semi-conducting materials, especially the III–V semiconductors such as GaAs materials.
There is a fundamental difference between the manner in which these MZI devices are produced in the two materials. The MZI devices formed of lithium niobate are produced with co-planar electrodes formed on what is conventionally shown as the upper surface of the lithium niobate material with the parallel waveguides which form the arms of the MZI device located in the lithium niobate material below and between the parallel electrodes. In contrast, because semi-conductor MZI devices are produced by laminar epitaxial growth. Optionally an electrode in such semi-conductor MZI devices may be grown into the base of the layer containing the waveguides. These designs are referred to here as back-electrode designs. This grown back-electrode of doped, conductive semiconductor material, works together with metallic surface electrodes to enhance the efficiency of the electro-optic interaction.
This fundamental difference in design is shown clearly in FIG. 3 of the paper “High Speed III–V Semiconductor Intensity Modulators by R G Walker IEEE Journal of Quantum Electronics, Vol. 27, No 3. March 1991 pp 657 to 667 [herein Reference 1]. This Figure shows the difference in structure between the co-planar electrodes of the lithium niobate MZI device and the back electrodes of the semiconductor device such as the GaAs/ALGaAs devices.
This paper also shows a further fundamental difference between the two types of MZI devices in that the co-planar electrode MZI device such as the lithium niobate MZI device can easily be arranged to be in parallel push-pull format, whereas the semiconductor MZI devices are normally only arranged in series push-pull. Parallel push-pull as shown in FIG. 3 of Reference 1, entails cross-connection of the two arms in parallel (conceptually each being represented by a capacitor), and driving them from a single RF source (voltage V/2) such that they work in anti-phase. Series push-pull as shown in FIG. 3 of Reference 1, entails series connection of the two arms (conceptually each being represented by a capacitor), and driving them from a single RF source (voltage V) such that they work in anti-phase.
Operation in push-pull is to be preferred in MZI modulators and there are also certain advantages in operating in parallel push-pull rather than in series push-pull particularly where the RF power rather than bandwidth is of prime concern and where the waveguide capacitance can be made very low by other means.
In lithium niobate devices, see the paper “A review of Lithium Niobate Modulators for Fiber-Optic Communications Systems” by Wooten et. al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 6, No. 1, January/February 2000 PP 69 to 82, FIGS. 3a and 3b, co-planar waveguide (CPW) electrode structures with a central ‘hot’ and two outer ‘ground’ rails are typically used to apply lateral E-field components to two waveguides from left to right and from right to left respectively. FIGS. 3c and 3d of this reference represent approaches in lithium niobate to series and dual-drive push-pull electrode configurations.
In III–V semiconductors lateral fields are not favoured in the normal <100> crystal-cut as this gives a polarisation-rotation effect rather than phase modulation. However, an arrangement of electrodes is possible, see the paper GaAs/AlGaAs electro-optic modulator with bandwidth >40 GHz, Electronics Letters, Vol. 31, No 11, 25th May 1995, pp 915 to 916, in which the E-fields are directed down through one waveguide and up through the other. This also is a type of parallel push-pull configuration but it is inefficient and the field lies neither perfectly vertical, nor laterally symmetric, and as a result parasitic polarization effects occur. Further this design is not a back-electrode design and as such it is not efficient, but such a design does not require isolation trenching.
Because of the different geometry of the two types of MZI modulators, only the MZI modulators of the co-planar type such as the lithium niobate based modulators have conventionally been able to be operated in parallel push-pull mode heretofore, and the back-electrode designs which are fabricated in semiconductor materials have been operated in series push-pull mode.
The present invention overcomes this limitation and provides a means for operating a doped semiconductor material based MZI modulator in parallel push-pull mode. Amongst other advantages, this is desirable because, compared to competing non-semiconductor materials such as lithium-niobate and polymers, semiconductor materials such as III–V materials are environmentally stable over temperature, time, and stress, with negligible D.C. drift. They also have a high optical power tolerance and a process-route that allows waveguides of differing properties to be combined on one substrate.
The present invention provides means whereby the high efficiency advantages of the doped, multi-layer III–V semiconductor waveguide may be retained in a parallel push-pull configuration.